I have lots of experience w/ allergies, w/ 2 of mine. I would not use Benadryl as it usually does not work well and you must give it every 4-6 hours for it to remain therapeutic. I'd use a 24hr med...most Vet Derms recommend Zyrtec or Claritin.
Read more about how to approach allergies here:
http://www.yorkietalk.com/forums/die...tart-here.html
For your kiddo though, I'd also be wondering about reflux, or, if something is stuck in the nose or throat:
http://www.yorkietalk.com/forums/die...wallowing.html